Expansion valves are commonly used for bringing a gas from its transport or storage pressure to its service pressure. Control over the impact that expansion valves have on the chemical composition of the gases they deliver is important because it is known that expansion valves can have an impact on the composition of the gases passing through them. This impact is a detrimental impact when using gases or mixtures of gases the composition of which needs to be known precisely, or the purity of which needs to be strictly preserved, such as gases in laboratories and analysis gases and the gases used in electronics. It may also have an impact on gases that may present a danger when mixed with other gases or with the ambient air: it is necessary to control their delivery in order to avoid any dangerous mixture. This is the case, for example, with fuel gases such as hydrogen or acetylene which may form explosive mixtures with ambient air. Contamination of the delivered gas also occurs when the delivery of the gas by the expansion valve is interrupted: the high-pressure upstream circuit of the expansion valve is closed, the low-pressure downstream circuit ceases to be supplied, and ambient air or other substances can enter the low-pressure circuit and the measurement and safety accessories.
Expansion valves are complex pieces of equipment comprising a high-pressure circuit and a low-pressure circuit which circuits consist of chambers and ducts, regulators (shutters, diaphragms, bellows or pistons), pressure gauges, safety means (valves, rupture disks, etc.). Known techniques for controlling the purity of the delivered gas are, for example:                the choice of sealing techniques, of materials used for the body of the expansion valve, seals, regulators (elastomeric diaphragms or metal bellows),        control of the quality of the surfaces, of the rinsing and passivation procedures and of the assembly conditions,        reduction of dead volumes,        the type of cleaning.        
In order to avoid the risks of contamination it is also possible to employ special gas delivery procedures (purging, rinsing by compression and expansion, operating procedure), but these are often lengthy and painstaking and operational errors are possible.
Finally, a gas nonreturn device may be installed at the exit from the expansion valve in order to avoid contamination of the low-pressure circuit by undesirable ingress, but this solution introduces a pressure drop in the delivery circuit and the sealing solutions used in such devices do not guarantee against microingresses of gas (107 mbar·l/s).